This invention relates to a process and system for treating waste with an anaerobic digester to produce methane and other valuable resources.
Methane digestion has been used for decades by sanitary engineers in the treatment of domestic sewage and organic wastes. Recently, its usefulness for waste processing has received new attention among farmers. Animal manures, garbage, and even refuse--all previously thought of as undesirable, troublesome "wastes"--can be digested under suitable conditions in order to produce valuable bio-gas and fertilizer.
The bio-gas generated by commonly used anaerobic waste digesters generally comprises a mixture of methane and carbon dioxide, with small additional amounts of other gases such as hydrogen sulfide. Though methane is an excellent fuel, bio-gas itself burns poorly. It is therefore desirable to separate the methane from the bio-gas mixture. Complex techniques have been used in the past to accomplish this separation, but a need exists for a simple, reliable, and inexpensive separation apparatus which is well suited for small scale operations.
One type of conventional anaerobic treatment system includes a digester tank adapted to contain waste and the bacteria responsible for the anaerobic process. Raw waste is introduced either periodically or continuously and is preferably mixed with the contents of the digester tank. The treated wastes and microorganisms are usually removed together as treated sludge. Sometimes this mixture is introduced into a second tank where the suspended material is allowed to settle and concentrate before the sludge is removed.
The sludge produced in a digester tank is a combination of the non-biodegradable portion of the solids introduced into the digester and the bacteria produced during the digestion process. The sludge contains nitrogen (pure and as ammonium ion), phosphorus, potassium, and trace elements, and is an excellent fertilizer and soil conditioner. However, if exposed to air, a great part of the nitrogen in the sludge is lost by the evaporation of ammonia. In addition, difficulties arise in using the sludge as a by-product of the digestive process because of storing and handling problems.
A number of factors play a significant role in the digestive process. The bacteria are sensitive to pH levels, temperature, and temperature variation. If conditions in the growing environment are not adequately controlled, the digestive process can take an excessively long time to finish and with poor results. Thus, these factors have an important effect on digester design and operation. These factors are particularly acute in small scale operations, in which the capital available for complex monitoring and control systems is limited.